The present invention relates generally to single frequency solid state lasers and, more particularly, to solid state lasers that operate at a single output frequency by extracting gain at this single output frequency from the entire length of the gain material.
In laser applications such as, for example, data transmission, telecommunications, and sensing, it is desirable to have a compact, low cost, efficient, solid state laser which operates at a single frequency. Unless special provisions are made, a conventional solid state laser operates at multiple frequencies, or axial modes, which compete for the available gain in the laser gain material. This phenomenon is known as spatial hole burning and results from standing wave patterns which selectively deplete the gain in only a limited portion of the gain material. Once a particular portion of the gain material is depleted, standing wave patterns corresponding to other, undesired frequencies can become active in the other portions of the gain material, thus giving rise to multimode operation of the laser.
A known method for achieving single frequency operation of a solid state laser involves insertion of intracavity elements, such as Lyot filters or Fabry-Perot etalons, within the laser cavity so as to induce differential loss among the various modes. The losses are at a level sufficient to extinguish the undesired modes, such that only one axial mode is permitted to lase while the undesired modes are suppressed, but are nonetheless present below their lasing thresholds (see, for example, U.S. Pat. Nos. 4,656,635 and 4,701,929 to Baer). One disadvantage of this method is the insertion loss attributable to the intracavity elements. In addition, the presence of undepleted inversion within the gain material lowers the efficiency of the laser. That is, the method does not take advantage of all of the gain available in the gain material. Furthermore, with regard to assembly concerns, the laser design becomes more complicated due to the need for the intracavity elements. Often, the resulting laser is difficult to align and sensitive to changes in environmental factors such as temperature.
Another prior art method used to attain single frequency solid state laser operation is the elimination of spatial hole burning to ensure that one axial mode uniformly depletes the available gain. Thus, the other, undesired modes are not merely suppressed below lasing threshold to avoid the possibility of their lasing, but are completely deprived of gain. The most commonly used methods of this type are the so-called "twisted mode" method (see U.S. Pat. No. 5,031,182 to Anthon et al.) and the ring laser method (see U.S. Pat. No. 5,052,815 to Nightingale et al.). In the twisted mode method, a combination of two quarterwave plates and a polarizer is used to circularly polarize the principal axial mode at the gain material to force the electric field vector of the principal axial mode to traverse in a helical pattern through the gain material, thus extracting the inversion uniformly. In a ring laser configuration, only one mode is allowed to oscillate by the use of non-reciprocal optical isolators. The unidirectional operation forces the principal axial mode to oscillate as a unidirectional travelling wave, thus uniformly extracting the available inversion and eliminating spatial hole burning. Unfortunately, the twisted mode and ring laser methods still typically require the presence of intracavity elements which, as mentioned above, introduce insertion losses, operation inefficiency, design complications, and sensitivity to environmental factors.
Still another prior art method for achieving single frequency output from a solid state laser by elimination of spatial hole burning involves physically moving the laser gain material back and forth within the laser resonator cavity. See H. G. Danielmeyer and W. G. Nilsen, Appl. Phys. Lett. 16, 124 (1970) (hereinafter Danielmeyer). This "gain sweeping" method, to a limited extent, mimics the behavior of gas lasers, where thermal motion of the atoms in the gain material essentially prevents spatial hole burning. Specifically, if the atoms of the gain material are in motion with respect to the nodal planes of the principal axial mode, the extraction of the available gain becomes spatially averaged through the gain material, thus yielding single frequency operation of the laser. Hence, in applying the method to a solid state laser, when the gain material is swept through the standing wave corresponding to the principal axial mode at a sufficiently large oscillation rate and oscillation amplitude, spatial hole burning can be eliminated and single frequency operation can be achieved. This method has been used to achieve stable, single mode operation of a flash lamp-pumped, Nd:YAG laser with a laser cavity length of 72 cm by moving the Nd:YAG rod, with an optical length of 5.5 cm long, by a longitudinal distance of .+-.15 cm at a rate of 0 to 10 cm/s.
It has also been suggested that a similar gain sweeping effect may be achieved by keeping the gain material fixed and somehow moving the nodal planes of the wave pattern corresponding to the principal axial mode. An electro-optic method for achieving this effect in a flash lamp-pumped, Nd:YAG laser is shown by Danielmeyer et al., Appl. Phys. Lett. 17, 519 (1970). Accordingly, a lithium niobate phase modulator is placed within the laser cavity at each end of the laser gain material to modulate the polarization state of the electric field corresponding to the principal axial mode. When the lithium niobate crystals are modulated at a predetermined frequency, with each crystal driven out of phase, the principal axial mode cannot establish a standing wave pattern, and hence the available gain is utilized efficiently.
The aforementioned electro-optic approach suffers a disadvantage in common with the differential loss, twisted mode, and ring laser methods in that intracavity elements are required. In addition, to avoid extra resonances, the surfaces of the lithium niobate crystals are typically held at an angle with respect to the light path, which leads to additional insertion losses.
The present invention introduces a highly advantageous and heretofore unseen single mode laser and associated method which utilize a swept gain extraction configuration.